1. Field of the Invention
The present invention relates to flywheel energy storage devices and, more particularly, to a large capacity hollow-type flywheel energy storage device that has improved stability and can minimize rotation loss.
2. Description of the Related Art
In general, power consumption largely varies between day and night in such a manner that it is very low at dawn but rapidly increases in the afternoon.
Thus, a power plant must be constructed to deal with the maximum load in the daytime irrespective of heavy economic loss. To level off the power consumption, a flywheel energy storage device is developed along with a pumping-up power generator, a device for midnight electricity, etc.
Such a flywheel energy storage device includes a motor generator, a flywheel and a bearing, and stores electric energy as inertial energy of the flywheel in a way of increasing a rotation speed of the flywheel using a motor to generate electricity using a generator connected to the flywheel, thereby enabling reuse of the electricity as needed.
Examples of the bearing for the flywheel energy storage device include a mechanical bearing, a superconductive bearing, a contactless electromagnet bearing, etc.
When using the mechanical bearing, the flywheel energy storage device operates not at high speed but at low speed, thereby providing small capacity. Further, friction loss is very severe during rotation. As a result, the flywheel energy storage device using the mechanical bearing has rarely been used.
When using the electromagnet bearing, the flywheel is floated by a repulsive force between an electromagnet and a permanent magnet. To maintain the floated state, the electromagnet must be precisely controlled. In this case, the electromagnet consumes a relatively large amount of energy, thereby lowering efficiency of the storage device.
When using the superconductive bearing, the flywheel is floated by floating and fixing forces between a superconductor and a magnet, and rotates at high speed without contact, thereby storing much energy in a small volume. However, a number of expensive superconductors are needed for floating the flywheel and eddy current loss increases in proportion to an area between the superconductor and the magnet. Thus, it is not easy to produce a large capacity flywheel energy storage device.
To solve these problems, there has been proposed a large capacity hollow-type flywheel energy storage device in which superconductive bearings and permanent magnet bearings are combined and disposed inside and outside a hollow shaft. The large capacity hollow-type flywheel energy storage device usually operates with a small rotation loss by minimizing the use of the electromagnet bearings and using the superconductive bearings to float a rotor. Here, only upon disturbance of the energy storage device, the electromagnet bearings are activated to suppress vibration and return to a state for minimizing power consumption.
In the large capacity hollow-type flywheel energy storage device, however, the electromagnet bearings and the superconductive bearings, all based on the magnetic force, must be spaced apart from each other in a longitudinal direction of the shaft to eliminate interference between the magnetic forces thereof when disposed inside and outside the hollow shaft.
Accordingly, the length of the hollow shaft is unnecessarily elongated to cause instability, so that rotation loss occurs due to a decrease in rotation speed, thereby lowering energy storage density.